1. Field of the Invention
The present invention relates to a case-hardening steel excellent in cold forgeability and low carburization distortion property.
2. Description of the Related Art
The steels used in gears, shafts, CVJ components and other such machine elements are generally case-hardening steels added with Cr and/or Mo. A machine element is manufactured by first cold-forging and machining the case-hardening steel to a predetermined shape and then carburization-hardening the steel. Cold forging offers good product surface finish and dimensional accuracy and achieves lower production cost and better yield than hot forging. Components conventionally produced by hot forging are therefore more and more being shifted to production by cold forging, so that in recent years the focus on carburized components produced by cold forging/carburization has increased considerably. Owing to this shift from hot forging to cold forging, reduction of steel cold deformation resistance and improvement of steel limit compressibility have become key issues. The former is needed for maintaining forging tool service life and the latter is needed for preventing steel cracking during cold forging.
To this end, Japanese Patent Publication (A) No. 2001-329339, for example, teaches a case-hardening steel for cold forging improved in cold forgeability by controlling C content to the range of 0.1 to 0.4% and controlling the shape of B-system inclusions. And Japanese Patent Publication (A) Nos. H11-335777 and 2001-303172 teach case-hardening steels for cold forging improved in cold forgeability by reducing Si and Mn content in a C content range of 0.1 to 0.3%, adding B to ensure hardenability, and further lowering bainite fraction.